<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .haizi2 {
            width: 100px;
            height: 100px;
            background-color: red;
            position: relative;
            margin: 100px auto;
        }

        .haizi2 div {
            position: absolute;
            right: -15px;
            left: -10px;
            width: 0;
            height: 0;
            line-height: 0;
            font-size: 0;
            border-bottom: 50px solid green;
        }
    </style>
</head>

<body>
    <div class="haizi2">
        <div>

        </div>
    </div>
</body>

</html>